Head up displays are known for motor vehicle use for displaying speed or other vehicle parameters in or adjacent to the line of sight of the operator to minimize distractions from the roadway view. By projecting a virtual image to the driver the gauge or other information will appear to be outside the vehicle, preferably at about the front of the vehicle. To accomplish this feat, an illuminated image source located beneath the vehicle dash pad is projected onto a reflecting element above the dash pad adjacent the operator view through the windshield. Indeed, the windshield itself is often used as the reflecting element, although windshield curvature introduces image distortions which must somehow be compensated for. It is also known to use a combiner which is a transparent sheet of glass or plastic above the dash pad just behind the windshield, and may be optimally shaped for image reflection, and also allows the roadway to be viewed through it. The reflecting element may also be a silvered mirror; in that case, it is not transparent and should be placed for minimal interference with the roadway scene.
Since the image projection requires an opening in the dash pad, it is feasible to retract the combiner or mirror when not in use and to restore it to operating position when the HUD is desired. Also, if the combiner or mirror is retracted, it becomes possible to cover the opening to keep out dust and other foreign material.
The position of the virtual image for a given optical configuration depends on the eye position of the operator. The image will appear to be higher or lower, depending on the operator's seating height, and it will appear to shift laterally if the operator moves to either side. If the reflecting element is large enough, the image will be visible from a large range of positions. However, to place the image at the position preferred by each operator, the HUD must be adjustable. This has the side benefit of permitting use of a smaller reflecting element.